(Not Applicable)
1. Field of the Invention
This invention relates generally to the preparation of high density foils, and more particularly to a method for preparing such foils from powders using high intensity radiant power.
2. Background of the Invention
The application of foils to various defense and commercial applications is well known. For example, the heat resistant properties of high strength metal alloys such as titanium aluminide make this material ideally suited for use in fighter jets. Other applications for high strength heat resistant composites include aerospace structural components, turbine engine components and optical mechanical components.
Presently, there exists many known refractory, composite and advanced intermetallic materials, such as titanium aluminide, which cannot be readily formed into foils by conventional foil fabrication techniques due to the extraordinary costs associated with the extensive thermal mechanical processing that is required by current processes. For example, a conventional ingot process to form 0.01 thick inch TiAl foil costs approximately $10,000 per square foot. In many cases, the extreme costs completely eliminate any commercial use of the material even though the material might possess outstanding properties in the area of interest. Some materials are inherently difficult to process. For example, intermetallics have very high work hardening rates and low room temperature ductilities making them difficult to process. Consequently, difficult to process materials such as intermetallics are currently only prepared for niche markets because known preparation methods are not cost effective.
Although some processing techniques exist to form foils such as processing used to form high-temperature fiber reinforced metal matrix composites (xe2x80x9cMMCsxe2x80x9d) foils, MMC foils appeal to a very limited market because such foils are expensive to fabricate and the fabrication is time consuming. Typically in an MMC process, ceramic fibers are coated with a metal matrix by a process such as plasma spray deposition or sputtering. Even if these processes perform well, they result in intermediate materials which are not useful in engineering applications, due in part to the presence of significant residual porosity. Substantial further processing, such as one or more hot pressing/diffusion bonding steps are generally required to compact these intermediate materials into reasonably dense components. Conventional powder techniques are also generally available for forming quality foils. However, conventional powder processing is also expensive and time consuming.
Conventional powder processing techniques typically involve the initial formation of a green sheet preform through the mixture of a powder and a polymer binder. The green sheet is placed in a vacuum furnace where the binder is burned off. Burn-off conditions may be chosen to result in some pre-sintering of the powder. A higher temperature sinter step is then performed to allow bonding of the powder material through solid state diffusion. The sheet is then rolled, re-sintered and this process of rolling and re-sintering is repeated several times. Vacuum annealing steps are required to relieve the stress produced from rolling the sheet. Typical sintering times are 13 hours for each sintering step. Thus, this technique may require several days to complete.
While the aforementioned techniques and several others are generally available for forming some foils with reasonable mechanical characteristics, they all share the undesirable characteristics of being time consuming and very expensive. A need exists for a method of preparing foils that is rapid, inexpensive and results in superior film properties. It would be advantageous for such a method to also be easy to control and adaptable for use with a wide variety of materials.
A method for preparing a high density foil which utilizes high intensity radiant power is provided. Initially, a green sheet preform is made by cold green casting a powder with a liquid suspension medium, such as a polymer, which functions as a binder. The green sheet is loaded into a process chamber. The green sheet is then exposed to a radiation source which emits wavelengths of radiation which are preferentially absorbed by the powder material, resulting in heating the green sheet and formation of a foil on a comparatively cold process chamber surface which acts as a cold hearth. The foil can then be cooled in the process chamber prior to removal.
If the binder material selected is a polymer, it may be reduced to residual carbon prior to exposure of the sheet to significant radiation with the addition of a preheat step. To produce a large area of uniform heat flux, the radiation source, the process chamber or both the radiation source and the process chamber may be translated relative to each other.
If the material of interest is susceptible to oxidation, non-oxidizing gases selected from the group consisting of hydrogen and noble gases may be used to create an inert atmosphere for processing. Carefully controlled radiation rapidly heats the surface of the green sheet while the temperature of the green sheet subsurface and the process chamber remains at substantially lower temperatures. The foil may be formed by either melting the surface of the green sheet or simply heating the surface below the melting point of the powder. The radiation intensity and time parameters are expected to depend on the material being prepared and the desired foil characteristics. If a rapid process is desired, radiation parameters are chosen to melt the surface of the sheet. In this embodiment, the molten layer diffuses into the non-molten layer allowing the foil formed to retain a flat form. As used herein, diffusion is defined to include liquid state diffusion as well as solid state diffusion. As an alternative embodiment, the green sheet can be sintered so that only solid state diffusion occurs. However, this method generally requires several sintering and rolling steps to form a foil with the desired level of densification. Foil densification levels are generally referenced to single crystal values. For example, a foil densification level of 98% represents a foil density equal to 98% of the density of the same material in single crystal form.
Power densities of the radiant source will generally be from approximately 0.3 to 3.5 kW/cm2 and the thickness of the foil formed is approximately 1 to 4000 microns. If the sheet is not fully dense after one pass under the lamp, the material may be cold rolled and passed back under the lamp. Cold rolling is frequently used when sintering alone is used to attain desired foil characteristics.
The process chamber may be made from aluminum. Aluminum is inexpensive and malleable. The process chamber is designed to have one or more openings to allow transmission of radiation inside the process chamber. Quartz may be chosen as a suitable radiation transparent material. The process chamber may include cooling structure to prevent melting of the process chamber. A circulating cooling fluid may be used to cool the process chamber.
In an alternate embodiment of the present invention, a method is provided for the continuous or near continuous formation of a foil. In this embodiment, a roll of green sheet is fed through a process chamber adapted to allow passage of the green sheet therethrough. Radiation processing and cooling are preferably performed within the process chamber.